Development Of Material Model Research Articles

A Modified Johnson–Cook Model for Dynamic Response of Metals with an Explicit Strain- and Strain-Rate-Dependent Adiabatic Thermosoftening Effect

Metallic alloys are extensively utilized in applications where extreme loading and environmental conditions occur and engineering reliability of components or structures made of such materials is a significant concern in applications. Adiabatic heating in these materials during high-rate deformation is of great interest to analysts, experimentalists, and modelers due to a reduction in strength that is produced. Capturing the thermosoftening caused by adiabatic heating is critical in material model development to precisely predict the dynamic response of materials and structures at high rates of loading. In addition to strain rate effect, the Johnson–Cook (JC) model includes a term to describe the effect of either environmental or adiabatic temperature rise. The standard expression of the JC model requires quantitative knowledge of temperature rise, but it can be challenging to obtain in situ temperature measurements, especially in dynamic experiments. The temperature rise can be calculated from plastic work with a predetermined Taylor-Quinney (TQ) coefficient. However, the TQ coefficient is difficult to determine since it may be strain and strain-rate dependent. In this study, we modified the JC model with a power-law strain rate effect and an explicit form of strain- and strain-rate-dependent thermosoftening due to adiabatic temperature rise to describe the strain-rate-dependent tensile stress–strain response, prior to the onset of necking, for 304L stainless steel, A572, and 4140 steels. The modified JC model was also used to describe the true stress–strain response during necking for A572 and 4140 steels at various strain rates. The results predicted with the modified JC model agreed with the tensile experimental data reasonably well.

Development of Calculus Material Model For Special Purpose of Accounting Expertise

This study aims to determine the functional relationship of calculus subject teaching material models that can encourage students’ interests, talents and learning motivation towards the achievement of accounting expertise in the Commerce Department of Politeknik Negeri Lhokseumawe (PNL). The sample of this study were 100 Accounting Study Program students from the Department of Commerce who were registered as second grade which had completed financial mathematics course. The data collection techniques in this study were using questionnaires and data analysis methods. The easiest process to understand accounting expertise is by using teaching materials of algebraic and integral differential calculus, specifically algebraic differentiation can be used anywhere especially in calculating interest. The results showed that firstly, the development of calculus teaching material models between student interest has been assumed to have a positive and significant effect on the GPA of Accounting Students, especially for the Diploma three of Accounting Study Program at the PNL. Secondly, the development of calculus teaching material model has a positive and significant effect on the student motivation of Accounting at PNL. Third, the development of calculus teaching material model for the specific purpose of accounting expertise, students’ interest and motivation are dominantly influent the GPA of the students.

Micromechanically-motivated analysis of fibrous tissue

Collagen fibers are the main load bearing component in fibrous tissues. Systematic analyses of their structure and orientation are thus crucial for the development of material models that enable to predict the mechanical tissue response. To this end, biaxial tests at different stretch ratios were performed on two tissue samples of the medial layer extracted from a human aorta. The tissues were loaded in the circumferential and axial directions simultaneously. We develop here a micromechanical model which is based on structural parameters of collagen fibers that were extracted from second-harmonic generation images of the two samples. The tissue is modeled as a periodic six-layered laminate in which the individual layers are treated as periodic fibrous structures with one family of fibers. We make use of the Hill-Mandel theory in the context of periodic homogenization to determine the overall mechanical tissue response. Both the analytical and numerical models are able to capture the overall mechanical response of the two tissue samples using a straightforward representation of the tissue structure together with a limited set of material parameters. Up to 10% of strains the model captures the almost linear response of both tissue samples. Beyond that stretch level the stiffening of the tissues becomes more evident, especially in the circumferential direction. In cases where the axial stretch is larger than the circumferential stretch the predictions are somewhat stiffer, while a very good agreement is obtained when the circumferential stretch is dominant. The stiffening of one tissue sample was substantially larger than the other, implying that higher-order stiffening mechanisms may kick in at larger strains. Our sensitivity analyses reveal that the parameters of the material model and the fiber dispersion have a minor effect on the tissue response. The novel modeling approach has the potential to reduce the need of time-consuming experimental data of the mechanical behavior of fibrous tissues.

Development of an Elastic Material Model for BCC Lattice Cell Structures Using Finite Element Analysis and Neural Networks Approaches

Lattice cell structures (LCS) are being investigated for applications in sandwich composites. To obtain an optimized design, finite element analysis (FEA) -based computational approach can be used for detailed analyses of such structures, sometime at full scale. However, developing a large-scale model for a lattice-based structure is computationally expensive. If an equivalent solid FEA model can be developed using the equivalent solid mechanical properties of a lattice structure, the computational time will be greatly reduced. The main idea of this research is to develop a material model which is equivalent to the mechanical response of a lattice structure. In this study, the mechanical behavior of a body centered cubic (BCC) configuration under compression and within elastic limit is considered. First, the FEA approach and theoretical calculations are used on a single unit cell BCC for several cases (different strut diameters and cell sizes) to predict equivalent solid properties. The results are then used to develop a neural network (NN) model so that the equivalent solid properties of a BCC lattice of any configuration can be predicted. The input data of NN are bulk material properties and output data are equivalent solid mechanical properties. Two separate FEA models are then developed for samples under compression: one with 5 × 5 × 4 cell BCC and one completely solid with equivalent solid properties obtained from NN. In addition, 5 × 5 × 4 cell BCC LCS specimens are fabricated on a Fused Deposition Modeling uPrint SEplus 3D printer using Acrylonitrile Butadiene Styrene (ABS) and tested under compression. Experimental load-displacement behavior and the results obtained from both the FEA models are in good agreement within the elastic limit.

Elastic-plastic deformation of molybdenum single crystals shocked to 12.5 GPa: Crystal anisotropy effects

To understand crystal anisotropy effects on shock-induced elastic-plastic deformation of molybdenum (Mo), results from high-purity single crystals shocked along [110] and [111] orientations to an elastic impact stress of 12.5 GPa were obtained and compared with the [100] results previously reported [A. Mandal and Y. M Gupta, J. Appl. Phys. 121, 045903 (2017)]. Measured wave profiles showed a time-dependent response, and strong anisotropy was observed in the elastic wave attenuation with the propagation distance, elastic limits, shock speeds, and overall structure of the wave profiles. Resolved shear stresses on {110}〈111〉 and {112}〈111〉 slip systems provided insight into the observed anisotropy in elastic wave attenuation and elastic limits and showed that shear stresses, and not longitudinal stresses, are a better measure of strength in shocked single crystals. Under shock compression, resolved shear stresses at elastic limits were comparable to the Peierls stress of screw dislocations in Mo. Elastic wave attenuation was rapid when shear stresses were larger than the Peierls stress. Large differences in the elastic limits under shock and quasi-static loading are likely a consequence of the large Peierls stress value for Mo. Numerically simulated wave profiles, obtained using the dislocation-based plasticity model described in the [100] work, showed good agreement with all measured wave profiles but could not differentiate between the {110}〈111〉 and {112}〈111〉 slip systems. Overall, experimental results and corresponding numerical simulations for the three crystal orientations have provided a comprehensive insight into shock-induced elastic-plastic deformation of Mo single crystals, including the development of a continuum material model.

A new one-phase material model for the numerical prediction of critical material flow conditions in thixoforging processes

Thixoforging allows one-step forming processes of near-net shape components having excellent mechanical properties. However, the high sensitivity of thixoforging regarding process conditions requires precise modelling and determination of process related parameters. At the same time, simple numerical design proves challenging because of the inaccuracy of existing one-phase material models regarding the shear thinning flow behaviour of semi solid metals. Consequently, this paper deals with the development of a new one-phase material model providing a more precise simulation of materials’ shear rate dependency. By using this model, simulations could be performed, which allowed the prediction of solidification and flow-related component defects.

Experimental and numerical characterization method for forming behavior of thermoplastics reinforced with woven fabrics

Abstract The automotive and aviation industry has to achieve significant weight reduction in order to fulfil legal obligations. This leads to an increasing use of new materials or new material combinations like fibre-reinforced plastics (FRP) as they provide a high lightweight potential due to the combination of low density and high tensile strength. Meanwhile pre-impregnated sheets with a thermoplastic matrix reinforced with woven carbon fibres are commercially available. This has led in a significant cost reduction and hence, the FRP have become affordable for large scale production. The material properties, in particular the forming and failure behaviour of the FRP, differ strongly from that of conventional metal materials like steel or aluminium. Therefore, new material characterisation techniques, investigation methods as well as numerical models are required. The main focus of this paper lies on the development of a non-orthogonal material model for the FRP, its implementation in a commercial FE-software as well as on the use of a combined experimental-numerical procedure for material characterisation. Since the properties of these materials are strongly temperature dependent, the forming process of reinforced thermoplastics is typically carried out at elevated temperatures. Thus, temperature sensitivity has to be taken into account during experimental testing as well as in the model approach. The model parameterisation is carried out based on an iterative numerical optimization procedure. For this purpose, the experimentally obtained results are investigated by means of digital image correlation and linked with the numerical model in combination with an automated optimization process.

Three stage creep behavior of MgO containing ordinary refractories in tension and compression

In service tensile and compressive stresses occur in refractory linings, these stresses lead to creep of refractories. Ordinary refractories experience creep of the primary stage and may further proceed to the secondary and tertiary creep stage. For the development of advanced material models for finite element simulations it is necessary to investigate the creep behavior in all three creep stages under tensile and compressive loads. Hence, two advanced high temperature uniaxial creep testing devices, applying a wide range of tensile and compressive loads, were used to determine the three creep stages in a reasonable time under service related loading conditions. The Norton–Bailey creep equations and an inverse identification procedure were applied for the evaluation of the experimental results. A magnesia refractory was studied at elevated temperatures and its respective creep parameters for each stage were determined. The stress dependency on the creep behavior can be seen clearly on the creep curves and the corresponding creep parameters. Furthermore, a comparative study of creep parameters and creep rates was performed between the magnesia refractory and a magnesia-chromite refractory. The results demonstrate the significant asymmetrical creep behavior in tension and compression for both materials. The creep investigation in this paper favors the requirement for consideration of the three stage creep behavior and the asymmetrical creep behavior in thermomechanical modelling activities of industrial vessels.

Explicit failure model for boron carbide ceramics under shock loading

Ceramic materials have a long-term industrial demand due to their high mechanical hardness and chemical and temperature resistance. They are brittle and tend to lose strength under heavy loads which complicates the development of a comprehensive material model for simulation of engineering prototypes containing ceramic parts. We developed an improved failure model of ceramics based on the well-known Johnson–Holmquist approach. This model redefines the damage rate equation using a consistent definition of the total plastic strain in the failed material. It reduces the number of free model parameters and enables the plastic strain to be explicitly accumulated during the failure process. The corresponding non-iterative algorithm utilizing this explicit failure model is developed. It is successfully validated by simulation of the wave profiles obtained in plate-impact experiments with boron carbide using the contact smoothed particle hydrodynamic method.

Material Testing in Support of the Development and Calibration of Material Models for Forming Simulations

Material testing at various strain rates, temperatures and loading conditions provides data that is used for the development and calibration of constitutive equations (material models) that are utilized in numerical simulations of sheet metal forming. In general, the testing can be divided into characterization tests and validation tests. In characterization tests basic material properties (e.g. yield stress, ultimate stress, failure strain) are determined from a test in which a material coupon is loaded under a well-defined condition (stress, strain rate, temperature, etc.). The data is used for determining the values of parameters in plasticity and failure models. In validation tests a material specimen or a small component is loaded with a more complicated, but well defined, loadings. The test is numerically simulated and the calculated quantities (forces, deformation, failure, temperature, etc.) are compared with measurements. The recent development of the Digital Image Correlation (DIC) technique for full-field measurement of deformation has extended the useful data that can be extracted from traditional characterization tests and provide means for developing new experiments that can be used for obtaining more accurate material models. This paper reviews the advantages of using DIC in traditional tests and presents several new recently developed testing configurations for material coupons and small components. Many of these tests have been used during in the development of a failure surface that gives the equivalent plastic strain to failure as a function of stress triaxiality and the Lode parameter.

A new material model for concrete subjected to intense dynamic loadings

The development of material models for concrete subject to intense dynamic loadings, e.g., impact, penetration and blast loadings, is a topic of current research. A high-fidelity physics-based concrete material model should not only capture the high strain-rate, high pressure and large strain generated by intense dynamic loadings, but also the typical failure phenomena of concrete material, e.g., spalling and cracking. This paper presents a new concrete material model, which includes the advantages and discards the disadvantages of frequently used commercial material models. In this model, a three-invariant failure surface is proposed by interpolating the maximum strength surface and the residual strength surface based on the level of current damage. The compressive damage and tensile damage are separately treated. The compressive damage is a function of the modified equivalent plastic strain, while the tensile damage is based on observations of recent dynamic tensile tests which demonstrated that, at high strain-rate, the fracture strain is a constant and the fracture energy increases with increase of strain-rate. Strain-rate effect is taken into account through the radial enhancement approach where the failure surface is enhanced equally in radial direction. To capture the shear dilation of concrete material that is ignored by most commercial concrete material models, a partially associative flow rule is introduced. Automated generation of parameters is suggested for convenience of practical application. The new concrete material model is implemented into the finite element code LS-DYNA through user defined material model. To validate the new material model, comprehensive single element tests including the uniaxial, biaxial and triaxial compressions and tensions are firstly conducted, where the improved performances are demonstrated by comparisons of corresponding predictions by the commercial concrete material models. Then selected numerical examples covered a wide range of loading intensity are presented, and numerical predictions are found to be in excellent agreement with corresponding experimental data.

Multi-scale investigation of highly anisotropic zinc alloys using crystal plasticity and inverse analysis

Zinc and its alloys are important industrial materials due to their high corrosion resistance, low cost and good ductility. However, the characterization of these materials remains a difficult task due to their highly anisotropic behavior, the latter being due to the influence of microstructural effects, i.e. loading orientation-dependent activation of different families of slip systems and subsequent texture evolution, rendering the development of a reliable material model considerably difficult. A micro-mechanical approach based on polycrystal plasticity would better describe the physical mechanisms underlying the macroscopic behavior. This improved model should ostensibly improve the comprehension of the mechanical behavior, compared to the macroscopic approach using solely phenomenological anisotropy models along with a prohibitively large number of experiments required to identify the material parameters. In this paper, a multi-scale Visco-Plastic Self-Consistent (VPSC) approach is used. It is based on a micro-scale model calibrated by microstructural and deformation mechanism information based on Electron Back-Scattered Diffraction (EBSD) to describe the macroscopic anisotropic mechanical response during sheet metal deformation. The critical resolved shear stress (CRSS) as well as the micro-scale crystal parameters are obtained by an inverse analysis comparing the simulated and experimental results in terms of obtained tensile curves along three different directions. In order to obtain a global solution for the identification, we then use the Covariance Matrix Adaptation-Evolution Strategy (CMA-ES) genetic algorithm to the inverse problem. We validate our approach by comparing the simulated and experimental textures and activated slip systems. Finally, the identified mechanical parameters are used to investigate the anisotropy of the alloy and predict its formability by determining the corresponding R-values and Hill yield coefficients.

MODEL MATERI AJAR KITABAH ARABIYAH BERBASIS PENDEKATAN PROSES GENRE DI PERGURUAN TINGGI

This study aims to obtain a model of teaching materials of Arabic writing (kitabah Arabiyah) based on the genre process approach at the State University of Jakarta. The method used is the development research to obtain the model of Arabic writing materials based on the genre process approach. The results showed that students still had difficulties at a fairly high level, such as finding key ideas, developing paragraphs, applying appropriate Arabic rules, finding and selecting the vocabularies that fit the context of the sentence. Students make some mistakes when writing Arabic script, including incorrect writing of letters, selection of verb (fi’il) and noun (isim), mistakes of intended meaning, incomplete sentences, and incomplete paragraphs. The need of students to the learning of Arabic writing shown by the scores of students' needs on the study is 3.90 which means that Arabic writing teaching is needed. The development of Arabic-based teaching material model based on the genre approach is needed as an alternative to problem solving.

ARABIC WRITING LEARNING MODEL BASED ON GENRE PROCESS APPROACH IN HIGHER EDUCATION

This study aims to obtain a model of teaching materials of Arabic writing (kitabah Arabiyah) based on the genre process approach at the State University of Jakarta. The method used is the development research to obtain the model of Arabic writing materials based on the genre process approach. The results showed that students still had difficulties at a fairly high level, such as finding key ideas, developing paragraphs, applying appropriate Arabic rules, finding and selecting the vocabularies that fit the context of the sentence. Students make some mistakes when writing Arabic script, including incorrect writing of letters, selection of verb (fi’il) and noun (isim), mistakes of intended meaning, incomplete sentences, and incomplete paragraphs. The need of students to the learning of Arabic writing shown by the scores of students' needs on the study is 3.90 which means that Arabic writing teaching is needed. The development of Arabic-based teaching material model based on the genre approach is needed as an alternative to problem solving.

Modelling multiple cycles of static and dynamic recrystallisation using a fully implicit isotropic material model based on dislocation density

This paper presents the development and numerical implementation of a state variable based thermomechanical material model, intended for use within a fully implicit finite element formulation. Plastic hardening, thermal recovery and multiple cycles of recrystallisation can be tracked for single peak as well as multiple peak recrystallisation response. The numerical implementation of the state variable model extends on a $$J_{2}$$ isotropic hypo-elastoplastic modelling framework. The complete numerical implementation is presented as an Abaqus UMAT and linked subroutines. Implementation is discussed with detailed explanation of the derivation and use of various sensitivities, internal state variable management and multiple recrystallisation cycle contributions. A flow chart explaining the proposed numerical implementation is provided as well as verification on the convergence of the material subroutine. The material model is characterised using two high temperature data sets for cobalt and copper. The results of finite element analyses using the material parameter values characterised on the copper data set are also presented.

Development of constitutive material model of 3D printed structure via FDM

The present paper develops the constitutive material models of the 3D printed parts via fused deposition modeling. Additive manufacturing of a part results in a complex microstructure which depends on the process parameters and build orientation. Consequently, anisotropy is introduced into the material properties. The mechanical behavior of the printed parts is governed by the constitutive behavior of the material. Therefore, the stiffness matrix of the material of the final printed part needs to be estimated for accurately capturing their behavior. The constitutive material modeling of the printed parts using numerical homogenization procedure is emphasized in this work. The present simulation models can capture the influence of build orientation, printing direction and layer thickness on the material behavior of the printed parts. Then, the influence of layer deposition in printing of differently oriented parts of the structure on the material behavior is investigated. It is revealed that the material behavior of different parts of the structure is not same and is dependent on the build orientation of the parts and also their thickness. This work aids the computation of elastic moduli and also selecting of the correct constitutive material model of the printed parts for stress analysis.

Introduction of materials modelling into metal forming simulation

Materials data have been an integral part of metal forming simulation. The essential data for such simulations are wide ranging, from physical and thermophysical properties to rheological properties as well as phase transformation kinetics. This paper reviews our development of material models over the past two decades, by which many of the material properties needed for forming simulation can be calculated. These models have been implemented into computer software, JMatPro®, and the calculated property data can now be easily transferred to CAE tools for the simulation of metal processing. A case study is presented here on the simulation of the hammer forging process of a 30CrMo4 steel using software Forge®. The simulated results demonstrate good agreement with experimental observations. Merging alloy design and processing design into one design loop now becomes a possibility.

Development of a constitutive model of soft tissues for FE analyses using a bottom-up approach

This paper presents the development of a two-scale anisotropic hyperelastic material model whose microstructure is motivated by the arrangement of soft tissues. In a bottom-up approach, we start at the microscale, identifying the components that are relevant for our model. These components are represented by simplified mechanical elements, such as linear springs and incompressible volumes. The next step is to use the concept of the representative volume element connecting the micro- and macroscales. Introducing principal material directions, the notion of invariants and pseudo-invariants is employed to derive a formula for the strain energy function. In fact, two hyperelastic models are proposed. In the simplified one, the microstructure is formed of a network of linear springs. In the second one, an incompressible volume is added to the representation of the microstructure. This results in the model’s having a nonlinear response, with the strain energy function arising as a solution to a minimization problem. The properties of the strain energy function and the influence of anisotropy are demonstrated on a simple tension test and a simple shear test. Applications of the proposed model to the description of prestressed materials, non-affine deformations, and real tissue modelling are presented.

Characterization and modeling of the fatigue behavior of TPU

A comprehensive determination of the material properties is of major importance for the development of an adequate material model for finite element (FE) calculations and further lifetime estimations. As a preparation for the FE simulations, the time-, rate-, temperature- and hysteresis dependency of TPUs is determined and translated into proper hyperelastic material models. The time dependency and the viscoelastic properties were determined with DTMA tests. The methodology of fatigue testing, particularly the determination of local strain bases Wöhler curve (LSWC) for TPUs on Diabolo specimens is covered. As a validation of the whole methodology, the lifetime of a simplified model of a rubber engine mount is calculated.

Numerical modelling of stainless steel preloaded bolted connections

AbstractThe use of stainless steel in construction has become more popular in recent years. It is used for a wide range of structural applications in aggressive environments where reliable performance over long periods with little maintenance is required. Although structural design standards are available for stainless steel, currently there are no rules covering the design of preloaded slip‐resistant bolted connections because of the lack of knowledge about their long‐term viscoplastic behaviour. Viscoplastic creep and stress relaxation in the preloaded bolt assemblies will lead to a certain loss of clamping force and may cause the failure of the connection if not accounted for. This paper presents the development of material models and finite element models for bolt assemblies based on an extensive experimental study of creep, relaxation and tension effects on austenitic, ferritic, duplex and lean duplex steel plates and bars for different loading rates. These models were verified against slip tests with stainless steel bolt assemblies according to EN 1090‐2 and then used in a parametric study to extend the scope of the connections investigated. Both experimental programmes were carried out in the European RFCS research project SIROCO (Execution and reliability of slip‐resistant connections for steel structures using Carbon Steel and Stainless Steel) as well as finite element calculations.